(a) Field of the Invention
The present invention pertains to a waveform producing system for use in, for example, an electronic musical instrument provided with waveform memorizing means which is utilized in tone generating circuits and/or tone modifying circuits including electronic keyers and keying circuits, and more particularly, it relates to a specific system for producing waveforms and having an aforesaid waveform memorizing means which is useful in, for example, the electronic musical instrument.
(B) Description of the Prior Art
In the conventional electronic musical instrument, a number of tone oscillators each having a different oscillation frequency are provided, whose output signals are synthesized in an appropriate manner to obtain a tone waveform. Such waveforms have been produced by another means, for example by passing a waveform containing a number of higher harmonics through filters of a complicated or sophisticated construction.
However, the conventional musical instrument which requires a number of tone oscillators or filters is too complicated to construct on a mass production basis and is difficult and troublesome to perform the adjustment of each oscillator and is not easy to obtain tone signals having desired complex waveforms. Thus, in the conventional instrument it has been hardly possible to produce natural sounds having complex and delicate waveforms resembling those produced by a natural musical instrument such as a guitar, piano and other instruments.
One conventional method for controlling or modifying the waveform of a keyed tone signal, that is, the prior method, is to provide the so-called tonal envelope characteristics to the keyed tone signal which characteristics include a build-up portion made upon the depression of a corresponding tonal key, a sustain portion and a decay portion made upon the release of the depressed key. Thus the method comprises, providing tone keyer circuits each having a charge-discharge circuit using a capacitor; applying a tone signal of a given amplitude to a corresponding one of the keyer circuits; and arranging an on-off switch provided in the charge-discharge circuit to coact with a key-operated switch. Thus, a tone signal with a predetermined envelope characteristic is obtained at the output side of the keyer circuits by the actuation of a single key. However, the charge-discharge circuit utilizing the capacitor cannot exhibit a complicated envelope characteristic which depicts a pattern consisting of an initially abrupt build-up curve with respect to the time axis, a steep descending curve at the extremity of the rise to a certain descending point and a subsequent slow decay curve, as is noted in conventional musical instruments. As a matter of fact, it has been considered in this field of art that any electronic musical instrument is acceptable for practical use only if it is provided with tone envelope characteristics substantially resembling those of the natural musical instruments even though the resulting envelope characteristics are somewhat monotonous and not sufficiently delicate.
As the tone control signal producing means for use in electronic musical instruments for producing a tone control signal in accordance with the depression speed of a playing key, there have been developed two kinds of so-called touch-responsive effect producing devices, one of which is of the type utilizing the charge-discharge function exerted by a combination of a capacitor and a resistor or resistors, and the other is of the type utilizing the induced electromotive force exerted by a combination of a magnet and a coil. In the case of the former type, it makes use of a delay curve which is determined exclusively by a time constant which, in turn, is determined by the product of the resistance of the resistor and the capacitance of the capacitor, and therefore the touch-responsive effect depends upon only the decay characteristic of the time constant, but this device is not always suitable for producing such tone signals as resembling those of the natural musical instruments at the output side of keyer circuits. Furthermore, the capacitor is required to be of a relatively large capacity in order to obtain a desired tone control signal responsive to the depression speed of the playing key, and so it has been difficult to make the tone control circuit of the instrument in the form of an integrated circuit. Because of the substantial time lag in the response between the changing-over action of a key-operated switch actuating the charge-discharge circuit and the resulting constitution of a discharge path, a maximum voltage stored on the charged capacitor cannot be derived at the output side of the tone control signal producing circuit, so that upon the maximum depression speed of the playing key, the utilization factor of the charged voltage to the tone control signal is low.
On the other hand, the aforesaid induced electromotive force type device is such that the movable magnet and the coil are arranged so that interlinked magnetic fluxes generated in the coil can vary depending on the depression speed of the playing key or the electromotive force or the voltage resulting from the interaction of the coil and the moving magnet is charged to a capacitor, thus utilizing a transient voltage variation across the capacitor during charging or a discharge characteristic of charged voltage through another discharge path. Thus, the touch-responsive effect may be said to be decided only by the factors of the coil and movable manget which are employed. This causes the same problems as mentioned above with respect to the case of the charge-discharge function type. In addition, the use of coils for generating an electromotive force makes the circuit integration of tone control circuits considerably inconvenient.